Antagonism of the subtype 3 of the sphingosine-1-phosphate receptors (S1PRs) is proposed to have therapeutic utility in asthma, chronic obstructive pulmonary diseases, as well as additional therapeutic utilities based upon receptor expression and the effects of pharmacological antagonism of gene deletion. Five high affinity G-protein coupled receptors for sphingosine 1-phosphate (S1P) are identified (1) and the crystal structure of S1PR1 has been solved (2). This cluster of receptors is medically important because the non-selective S1PR agonist fingolimod is an effective oral therapy for the treatment of relapsing-remitting multiple sclerosis by altering lymphocyte function. Various S1P receptor subtypes that differ in spatial distribution, coupling and function can singly or in combination, play complex roles in embryonic formation of the arterial media, blood pressure regulation and cardiac function. FTY720 (fingolimod) in man is associated with significant sinus bradycardia, heart block and a prolongation of QTc interval (3, 4). Atropine reversal of the sinus bradycardia (5) and the demonstration of sinus bradycardia with S1PR1-selective agonists in man (6) as well as rodents (7) suggested that sino-atrial (SA) node effects and those events resulting from alterations in ventricular conduction are distinctly regulated. Mice deficient in S1PR3 are resistant to a variety of pharmacological effects produced by agonists of S1PR3 including pulmonary and cardiac fibrosis (8-10), cardiac arrhythmias (11) as well as being resistant to complex pathologies such as cytokine storm and sepsis syndrome.
Sepsis syndrome, a consequence of infection and characterized by a state of uncontrolled systemic inflammation, kills approximately 200,000 people per year in the US (12, 13). According to global estimates, the incidence of sepsis is believed to range from 140-240 cases per 100,000, with fatality rates as high as 30%. If associated with circulatory collapse and end-organ failures, fatality rates remain in 50-80% range (14, 15). The 1979-2000 epidemiologic sepsis study estimated a 17 billion annual cost of sepsis care in the US (16), a value that is likely to be higher today due to the increased cost of healthcare. Although early intervention and modern supportive care practices in sepsis have slightly increased in overall sepsis survival rates, to 37 to 30% (17-21), there is still an obvious unmet medical need that requires development of new therapeutic strategies to combat this healthcare burden.
Despite measures to alter pathogen burden, and intensive supportive care, sepsis syndrome has high morbidity, mortality and a significant cost burden, reflecting imbalance between pro-inflammatory cytokines and elements of inflammation essential for host protection (22). Recent work defining the signature for key elements regulating systemic inflammation, has defined new, chemically tractable targets for therapeutic intervention that are genetically validated in animal models. Our recent work has demonstrated that blunting not abolishing host responses and cytokine storm provides important protection from immunopathology while sparing antiviral immune responses (23-25). In bacterial infections we have now demonstrated by both genetic deletion of receptor (26), as well as with the use of early selective, neutral antagonists, that S1P signaling via S1PR3 on dendritic cells (DC) exacerbates systemic inflammation and lethality in stringent models of sepsis, i.e. both LPS-induced inflammation and in cecal ligation puncture (CLP) models.
Sepsis syndrome is a significant unmet medical need, as no effective treatment options exist beyond antimicrobial therapies and supportive intensive care. Behind this medical challenge lie multiple, complex pathological endpoints that coalesce in final common pathways of end-organ failure, and prospective identification of patient subsets is a work in progress. None-the-less, the importance of the unmet medical need, coupled with new mechanistic insights into shared critical pathways, offers new opportunities for mechanism-based interventions. Characteristic pathological symptoms of severe sepsis include profound inflammation, dysregulated coagulation, tissue microvascular edema, cardiovascular collapse, renal dysfunction and ultimately death. An additional long-term consequence is pulmonary fibrosis. These symptoms result primarily from the hyper-activation of the host's immune system reacting to the pathogen's invasion (27, 28). Understanding the factor(s) regulating the onset and progression of the host's immune overactivation is relevant for designing novel effective therapies for sepsis. Multiple lines of evidence support crucial roles for S1PRs in the control of immune cell trafficking and cardiovascular functions in physiology and disease (29, 30). S1P, a circulating bioactive lysophospholipid derived from the ceramide pathway binds to and activates five closely related G-protein coupled receptors, referred to as S1PR1-5. Interestingly, human diseases with an active inflammatory component, such as multiple sclerosis (MS), coronary atherosclerosis, and lupus, have elevated plasma or local S1P levels (31-34). In the case of sepsis, there is even plasma elevation of a major S1P carrier lipoprotein, Apoprotein M, in disease subjects, and is now a risk factor for poor prognosis (35, 36). Thus it is likely that S1P signaling tone is consequently altered in septicemia. Since discontinuation of Xigris (37), an intended target of the endothelial components of sepsis, and since immunosuppressive corticosteroidal therapy can be controversial due to adrenal insufficiency occurring in sepsis (38, 39), there is a limited arsenal to combat sepsis. Inhibiting, with a systemic selective small molecule antagonist, S1PR3 on DCs, on vascular smooth muscle, coronary artery smooth muscle and bronchial smooth muscle can contribute to improving the therapeutic outcome in multiple clinical syndromes characterized by bronchoconstriction, pulmonary fibrosis, coronary artery constriction, cytokine amplification by dendritic cells, as well as the generation of disseminated intravascular coagulopathy, based upon data showing that S1PR3 signaling contributes to pro-inflammatory signals, fibrosis and to poor sepsis outcome.
Previous findings indicated that S1PR3 deficient DCs (taken from S1PR3 knockouts), significantly enhanced the survival of mice administered with a 90% lethal dose (LD90) of LPS or in mice following the Cecal Ligation Puncture (CLP) model of polymicrobial sepsis (26). Most importantly, the study pointed out that treatment with AUY954, a selective S1P1 agonist that sequesters B- and T-lymphocytes from the blood (40), and is useful for dampening inflammation in animal models of localized inflammation (41), did not infer any protection in the same study. Another report using similar transfer methods has just shown that S1PR3-deficiency in DCs significantly blunted pro-inflammatory mediators in renal ischemia/reperfusion studies and lowered kidney immunopathology in mice (42). Interesting, the authors further implicated IL-4 signaling as a downstream mediator of the S1PR3 deficiency benefits in renal ischemia/reperfusion. Furthermore, siRNA knockdown of S1PR3 in bone marrow derived DCs (BMDC) greatly reduced transwell DC migration, and migration to the mesenteric lymph node (43), suggesting that S1PR3 is directly involved in DC migration. Overall, the available evidence strongly suggests that down-modulating S1PR3 DC signaling, as proposed with a systemic S1PR3 antagonist, may open a new therapeutic opportunity in sepsis syndrome. These data strongly suggest that an S1PR3 antagonist may be valuable during the early management period of sepsis care, characterized as the critical therapeutic window with potential for boosting survival (44) (45).